A NEW MULTIPLE-EFFECT DISTILLER SYSTEM WITH COMPACT HEAT-EXCHANGERS

One of the goals of development in desalination distillation is to red uce the dimensions of the heat exchangers. Particularly in multiple-ef fect distillation the size of the evaporator tube bundle affects other design aspects like shell configuration, film generation, height of a stack-mounted unit. Heat transfer areas are essentially determined by energy and fouling considerations. All previous attempts to reduce si zes by sacrificing the smooth tube design had to be abandoned in large scale plants for cost reasons. Some years back DVT developed a compac t plate-type heat exchanger-the ''bavex'' heat exchanger. The plates o f this heat exchanger are corrugated and fitted together such as to ma ke it particularly suited for film evaporation. The heat transfer coef ficients are higher than with tubes and the evaporator volume per unit heat transfer area is less than 60% of the horizontal or vertical tub e film evaporator. The standard plate material for desalination applic ation is titanium. The ''bavex'' heat exchanger was adopted to provide compact evaporator cells for a low energy desalination distiller. A f irst standard unit with a capacity of 2500 m3/d has now been fully dev eloped and designed. It consists of a 12-effect stack-mounted module i n series at the warm end with a 4-effect horizontal package arrangemen t operating with heat recovery by thermo-compression across these firs t four effects. With top brine temperatures below 85-degrees-C, this c onfiguration achieves a performance ratio of 20. The philosophy of the development was to provide a variable system that can also be operate d with low temperature heat recovery or with waste heat from diesel ge nerators or gas turbines. The two modules can also be employed individ ually with a performance ratio 10. Another configuration which integra tes an absorption heat pump into the stack-mounted module can be shown to achieve performance ratios between 20 and 24 with max. brine tempe ratures below 70-degrees-C.